Method of purifying phosphate rock containing iron impurities



Feb. 24, 1953 L. E. DUPONT ET AL 2,629,650

METHOD OF PURIF'YING PHOSPHATE ROCK CONTINING IRON IMPURITIES Filed May 1:5, 1947 Patented Feb. 24, 1953 UNITED STATES PATENT OFFICE `METHOD OF`PURIFYING PHO SPHATE vROCK CONTAIN ING IRON fIMPURI-IIES York y zApplicationMayl, 1947, Serial No. 747,794

'1 The present invention relates to Vimprovements in the art of treating phosphate-bearing minerals-such Las phosphate rock and sand, so `as to eliminate iron which, if allowed to remain in the mineral, would'impair its usefulness for various purposes. v'Ihepresent application is a continuation in part of four pending application Serial No. 484,108, now abandoned, which was iiled on April 22, 1943,and'which` is Aassigned to the assignee of the 'present invention.

A further fobject of theinvention is to effect Ianfirwrease in the content of the ,usefulgor available 'ingredients `or chemicals in ,the `mineral` For examplephosphaterock obtainedfrom various deposits in Tennessee 'is comparatively low inTB4P. L. (boneiphosphateof lime or the normal phosphate of lime CagKPOfQz), "and itis very desirable that .the B. P. lL. of such rock should be raisedin'order that the iinishedproduct may be of greater economic'value than similar rock o having arelatively lowe1'.B.P. L. content.

Our improvedprocess .makes use of the A-prin-- ,ciple involved in the following reaction:

FesO3+GHClzZFeCls-IfI-IzO-FSZBG calories (exothermic) `nized.

^1n the experiments -and tests which we have madeinanieifort `to `apply the LAaforesaid reaction tothe commercial puricationof phosphatelminerals, we have .discoveredthat the followingprinciples or considera-tions are important or essential:

(l) The `reaction` gas which will give -up sufiicient chlorine to `satisfy the requirements of the iron should be in excess. vIn order to satisfy this requirement, it is desirable .for commercial reasons tc recoverthe vunreacted reaction gas for realise .in .the system.

(2) ln order to prevent undesirable or harmful side .reactions with the ferric .chloride in the reaction zone, or elsewhere in the system, oxidiz ing conditionsl must be avoided sofar as possible.

This consideration makes it desirable to operate the process ,as a .elosedsystem from which air substantially excluded.

(3) In lorder to insure .a satisfactory reaction, the temperature should be over about 315, as the 'ferrie chloride which results from'the reaction isavapor at these'temperatures and may be conveniently .conducted away'from thereaction Zone. However, vin the case of phosphate rock, the use 'of 'a temperature over about 400 C. is undesirable because highertemperatures alter the .physical structure of '.therock to the point that the treated rock may'notbe used in theproduction of superphosphate or other similar Valuable phosphate compounds.

(4) In order to obtain efficient operation and a high yield coupled with areasonably complete limination of the -unwanted impurities, it is 'necessary'to havethemineral reduced to a'relatively small particle size, and also there should -bea very intimate contact between the reagent -'gas and the mineral'particles within the reaction zone.

(5) Preferably, to prevent undesirable 'reactions'betwe'entheA ferrie chloride which is 'formed bythe reactionfand thetreated or untreated mineral, ,the `gaseous ferrie chloride should be conductedpromptly iaway from the 'treated or untreated mineral and, of course, out ofthe'reaction zone.

(6) All parts of the equipment with which any :corrosive 'gases or other materials may contact mustbe'constructed of refractory materials which will not be affected by Such corrosive gases or materials. Furthermore, the equipment must be sealed elfectivelyagainst the escape of corrosive .vapors and against the entrance of air, which .as above pointedout, would adversely affect the operation of theprocess Various types of equipmentrnay be used for 'carrying out the process, but of all the types of equipment whichwelhave used, We obtained the best results withian inclined rotary kilnprovided withsuitable meansfor feeding the comminuted mineral downwardlythrough the kiln .and for conducting the Areaction gas upwardly through .the comminuted mineral. Also, itis advisable to equip the apparatus with some form of internal heating device, for example a suitable electri- Acally heated refractory rotarycore element which, -on the exterior thereof, may also be equipped with -aworm or other suitable arrangement for effecting a downward propulsionof the mineral,

In using-such equipment, it is advisable to heat Athe comrninuted `mineral before -it is fed into the kiln-so as to reduce to a minimum the amount of `internal heating which may be required. Also, innrder toeifect proper'contact between the reaction -gas :and `the comminuted Vmineral, it is advisable to keep the kiln fairly full for the lower part of its length, the exit at the lower end being suitably controlled in accordance with the various conditions so as to maintain the required level of material within the kiln. Any suitable arrangement f sealed discharge may be employed in order to withdraw the treated material without permitting any substantial amount of air to enter the system, and in this connection, it is advantageous to keep the pressure at the discharge end of the kiln at about the same as that of the atmosphere, so that there will be no substantial pressure differential and consequent escape 0f gas or ingress of air in the event that the sealing arrangement is not perfect.

The principles and considerations which have been previously stated as being important or critical t0 the progress of the reaction will now be more fully described.

Insofar as we know, hydrogen chloride gas is the only reaction gas which may be successfully employed with phosphate minerals. This is due to the fact that the reaction gas must lsupply large amounts of chlorine for the reaction and the temperature of the reaction must be below about 400 C. to prevent injury to the structure of the phosphate material. Where hydrogen chloride gas is used as the reagent and the iron appears in the mineral as ferric oxide, the reaction is as follows:

(at over about 300 C.)

As before stated, it is desirable that the reagent be in excess both to increase the rate and eiectiveness of the reaction and to avoid oxidizing condition. We have discovered that it is critical to the operation of the process of the present invention that the concentration of the hydrogen chloride gas in the reaction zone be maintained at a value of at least about '70 percent by volume. If the concentration of the hydrogen chloride gas falls below 70 percent by volume, or if the volume of other gases including air are in excess of percent by volume, the removal of iron becomes an extremely slow and expensive procedure.

Ferrie chloride is vaporized at about 300 C., so that the temperature in the reaction zone must be kept over this value if the ierric chloride is to remain a vapor. We have discovered, however, that the practical lower temperature limit for iron removal in the process or the present invention is about 350 C. We have also discovered the upper temperature limit for the process of the present invention to be about 400 C. and the preferred operating temperatures to be in the range between 350 C. and 380 C. While higher temperatures, above 400 C., remove iron, they tend to calcine the rock, e. g. carbonate materials present in the rock, and to alter the physical structure of the rock so that after treatment the rock is unsuitable for further processing, as for example, conversion into superphosphate, for use in soil fertilizers, etc.

We have also discovered that the rock being treated should be comminuted to a degree such that it passes a mesh screen. and is retained on a 65 mesh screen to insure efficient utilization of the hydrogen chloride gas (the screen sizes referred to in the description and claims are Tyler Standard Screen sizes). Larger sized particles do not expose suiiicient area to the reagent gas or allow the required penetration of the reaffent gas into the particle, and extremely small particles pack so closely together that the reagent can- A not penetrate the mass of material being treated.

The period of time that the phosphate mineral remains in contact with the reagent gas is, as would be expected, also important. The results of a series of tests on the process of the present invention. in which the rate of material fed to the reaction kiln was varied, show that the time required for a reasonably complete removal of iron from the rock without effecting undesirable changes in the rock structure is from about 20 to 30 minutes. This time was determined for the preferred conditions of 350 to 380 C. temperatures, 35-65 mesh, particle size, and a reagent composition of over about 70 percent hydrogen chloride by volume.

In the drawing accompanying this application, we have illustrated in somewhat diagrammatic form an apparatus which was successfully used in carrying out the principles of this invention. In the apparatus, it will be observed that the phosphate rock -which has previously been crushed to about between 35 and 65 mesh is introduced into the hopper I. To the bottom of the hopper I, there is flexibly connected a pipe or conduit 2 which is vibrated by a conventional type of vibrator motor 3, so that the crushed rock will feed uniformly down through the conduit 2 and into the center of the right-hand end of a rotary pre-heater 4. The rotary heater 4 is equipped with a large end gear 5 which meshes with a small pinion 6 on a shaft which is driven at slow speed through a reducer 'I by a motor 8.

The kiln or heater 4 is supplied with heat by a stationary electric heating element 9 extending into the center of the kiln 4 and supplied with current through terminals I0 and II. The exit end of the heater or kiln 4 which may, if desired, be inclined slightly down towards said exit end, is equipped with a hood I2 into which the delivery end of the kiln 4 extends for a distance as shown.

The heated rock which passes out of the end of the barrel of the kiln 4 flows into the end of an inclined pipe I4 equipped with a suitable gate I5 for regulating the flow, and the lower end of said pipe I4 extends into a stationary housing IS having a flange I'I in which rotates the upper end of the reaction kiln, so that a reasonably gas-tight seal is effected between the stationary housing IS and said reaction kiln I8. As shown, said reaction kiln is quite inclined so as to effect the delivery of the material from right to left in the direction of the arrow I9.

The reaction kiln is heated by a cylindrical electrical resistance heating element 20 which extends throughout the length of the barrel of the'reaction kiln and rotates therewith. The current for supplying said electrical resistance element 20 is collected by a pair of rings 2| and 22 contacting with stationary brush elements respectively connected to conductors 23 and 24.

At the delivery end of the reaction kiln I8, there is arranged a stationary housing 25 having a iiange 25 which cooperates with a ange 21 on the lower end of the reaction kiln to provide a reasonably gas-tight joint as the reaction kiln rotates relative to said housing 25.

At the lower end of the delivery housing 25, there is arranged a spout or cone 2S which delivers into one end of a housing 29 within which housing rotates a suitable screw conveyor or worm 30 driven in the proper direction by a pulley or gear 3I so as to deliver the finished, treated ore out through the exit duct 32.

The reaction gas (hydrogen chloride) is introduced through the duct 33 and controlling valve 34 into the suction of a pump 35 which delivers into the exit housing 25 of the reaction kiln. Any gas which is produced in, or flows through, the reaction kiln is discharged through a gas exhaust pipe 35 from which it flows into a cooling tower 9'! in which any FeCla produced in the reaction kiln is collected. Surplus reagent flows out of the cooling tower 31 through a duct 3'8 controlled by a valve 39 and leading into a pipe 40 which connects with the lower` end of the pipe 33 leading into the inlet of the pump 35 previously described, so that said surplus gas is in effect recirculated through the kiln.

Condensation of the FeCla which enters the cooling tower 3l is facilitated by the use of a cooling jacket 4l lled with water or other coolant which is disposed about the tower 37. Said cooling water is supplied by a pipe 42 connected to a suitable supply of water. The flow of cooling water is controlled by a valve 43 so that the gas from the reaction kiln is cooled to below about 300 C. in order to insure the complete condensation of the ferrie chloride. The water is conducted to a suitable drain system by means of the pipe 44 which includes a valve 4'5. When necessary a relief valve or vent 46 may be employed to prevent undue pressure from accumulating in the system due to formation of unused gases. The condensed ferric chloride is conducted from the cooling tower 31 by means of a valve 45 and a pipeline 41.

Phosphate rock was treated in the equipment just described under the conditions which we have found critical for commercial and practical operation. The material feed analyzed 68.0% B. P. L., 5.10% FezOs, and 4.20% CaCOa, the feed was comminuted so that 95% was in the critical size range of 35 to 65 mesh (Tyler Standard Screen). The comminuted rock was preheated to 350 C., approximately the reaction temperature and the rate of feed into the reaction kiln was such that the comminuted rock remained in the reaction zone for about from 20 to 30 minutes where the temperature was maintained in the range between 350 and 380 C. The reagent, hydrogen chloride gas, was kept over about 70% by volume and the products of the reaction were rapidly removed and condensed.

The product after treatment as above analyzed 71.80 B. P. L. and 2.50% Fe203, and 3.04% CaCO. The B. P. L. of the nal product was raised 3.8% and the iron was reduced by about 51%. When this product was treated with sulfuric acid in the conventional manner a good textured superphosphate resulted.

The scope of the invention should be determined by reference to the appended claims.

We claim:

1. The method of purifying phosphate rock containing iron impurities, which comprises: exposing the rock in ccmminuted form to gaseous hydrogen chloride containing at least about 70 percent of hydrogen chloride by volume at a temperature between about 350 and 400 C., whereby the iron is separated from the rock in the form of ferrie chloride vapor, withdrawing the ferrie chloride vapor and the excess hydrogen chloride from the reaction zone, and separating the ferrie chloride from the hydrogen chloride.

2. The method of purifying phosphate rock containing iron impurities, which comprises: exposing the rock in comminuted form to gaseous hydrogen chioride containing at least about percent hydrogen chloride by volume at a temperature between about 350 and 400 C., whereby iron is separated from the rock in the form of ferrie chloride vapor, withdrawing the ierric chloride vapor and the excess hydrogen chloride from the reaction zone, and separating the ferrie chloride from the hydrogen chloride, the cornminuted rock under treatment having a substantial part of its particles between about 35 and 65 mesh.

3. The method of purifying phosphate rock containing iron impurities, which comprises: exposing the rock in comminuted form to an excess of gaseous hydrogen containing at least about '70 percent hydrogen chloride by volume at a temperature between about 350 and 380 C., whereby iron is separated from the rock in the form of ferrie chloride vapor, withdrawing the ferrie chloride vapor and the unreacted hydrogen chloride from the reaction zone and separating the ferrie chloride from the hydrogen chloride, the comminuted rock under treatment having a substantial proportion of its particles between 35 and 65 mesh.

4. The method of purifying phosphate rock containing iron impurities, which comprises: exposing the rock in comminuted form to an excess of gaseous hydrogen containing at least about 70 percent hydrogen chloride by volume at a temperature between about 350 and 380 C. for a period from about 2G to 30 minutes, whereby iron is separated from the rock in the form of ferric chloride vapor, withdrawing the ferrie chloride vapor and the unreacted hydrogen chloride from the reaction zone, separating the ferric chloride from the hydrogen chloride, and recycling the unreacted chloride through the reaction zone, the comminuted rock under treatment having a substantial proportion of its particles between 35 and 65 mesh.

LOUIS EUGENE DUPONT. JAMES ROBERT ARCHER. WILLIAM FLETCHER, CARTER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 991,096 Schroder May 2, 1911 1,129,407 Lay Feb. 23, 1915 1,147,832 Kugelgen July 27, 1915 1,406,595 Wescott Feb. 14, 1922 1,845,342 Saklatwalla Feb. 16, 1932 1,979,280 Mitchell Nov. 6, 1934 2,070,161 Flinn Jan. 2l, 1935 2,290,843 Kinney July 21. 1942 

